1. Field of the Invention
This invention relates to improvements of a magnetic recording medium used for a magnetic recording and/or reproducing apparatus, and it particularly relates to a coated-type magnetic recording medium having an excellent durability and surface flatness capable of suppressing development of uneven wear minimal on a tape contact surface of a magnetic head.
2. Description of the Related Arts
Presently, a magnetic recording medium such as a magnetic tape used for video tape recorders (referred to as VTR) or audio tape recorders is constructed by coating magnetic paint on a surface of a base film to form a magnetic layer on which information signals are to be recorded by magnetic heads. For the purpose of improving durability and abrasive resistance of the magnetic layer of the magnetic tape, non-magnetic particles selected from Al.sub.2 O.sub.3, Cr.sub.2 O.sub.3 and .alpha.-Fe.sub.2 O.sub.3 particles each having a Mohs hardness of more than 6 are added in the magnetic layer as abrasives or abrasive resistance additives.
As well known, these abrasives are effective to improve the durability and abrasive resistance characteristic of the magnetic tape, on the other hand, they polish and abrade a tape contact surface of the magnetic head. Thanks to the abrasive ability of the magnetic tape, such stains and foreign matters closely adhered to the tape contact surface of the magnetic head as magnetic particles along with binder separated from the magnetic layer are effectively removed by running the magnetic tape containing the abrasives along the tape contact surface thereof.
Along with magnetic recording mediums employing conventional oxide iron powder as magnetic powder, a ferrite magnetic head has been widely used as a recording and reproducing magnetic head so far. However, upon emergence of metal tapes having a higher coercive force (Hc), a metal type magnetic head (referred to as a metal head hereinafter) such as a Metal in Gap type head (referred to as MIG head), a metal laminated type head or a thin film type head is used in place of the ferrite magnetic head. Usually, the tape contact surface of the metal head is composed of a plurality of different materials each having a different hardness from one another. When the metal head is used for the magnetic tape containing the conventional abrasive made of particles each having a high mohs hardness such as alumina powder of which particles have a high .alpha.-rate (.alpha.-rate designates a degree of crystallization as mentioned hereinafter), the metal head tends to suffer from uneven wear in which wear occurs locally in metal materials having a lower hardness than those of other materials on the tape contact surface. Thus, the output level of the metal head is decreased upon reproducing recorded signal from the magnetic tape because of a spacing loss due to the uneven wear. Further, head cracks may be produced in the tape contact surface in the worst case.
Specifically, in the conventional ferrite head, the tape contact surface is composed of bonding glass portions and ferrite core portions which have a higher abrasive resistance than that of the bonding glass portions. Thus, even when the uneven wear occurs on the tape contact surface, the amount of wear is larger in the bonding glass portions than in the ferrite core portions, thus, the uneven wear does not prevent the ferrite core portions from contacting the magnetic tape closely. Therefore, degradation of the output level is negligible small in the ferrite head, if the uneven wear occurs.
On the other hand, in the metal head such as the MIG head, a magnetic gap which plays the most important role in recording and reproducing operation is formed between the metal cores. Thus, the metal core portions forming the magnetic gap therebetween readily wears compared with the other parts than the magnetic gap portion owing to lower hardness of the metal portions, which invites degradation of the output level of the metal head because of the spacing loss occurring due to a space between the magnetic gap and the magnetic tape.
Conventionally, in order to decrease the uneven wear produced on the tape contact surface, the abrasive is added into the magnetic layer of the magnetic tape so as to abrade equally the tape contact surface of the magnetic head so far. However, this causes a high degree of wear of the magnetic head, resulting in shortening the life of the magnetic head. When an abrasive made of smaller sized particles is employed in the magnetic layer to decrease the abrading ability of the abrasive, dispersiblity of particles of the abrasive is degraded. This causes the deterioration of the surface flatness of the magnetic tape, resulting in degradation of the electro-magnetic transfer characteristics, in addition, inviting the damage of the tape contact surface of the magnetic head upon operation.
In order to improve the dispersiblity of the small size particles, dispersant may be added into the magnetic paint, however, this dispersant has drawbacks that when the amount of the dispersant added is too small, the particles of the abrasive do not disperse in the magnetic paint throughly, and when the amount of the dispersant is too large, the dispersant seeps out on the surface thereof, i.e., a blooming phenomenon occurs on the surface of the magnetic tape.